Conventional methods for measuring cognitive load, include:                subjective measures, such as self-rating scales;        physiological techniques, such as pupil dilatation, heart rate and galvanic skin responses;        task or performance based measures, such as critical error rates and task completion times; and        behavioural measures, such as speech disfluencies, self-talk etc.        
There are a number of problems with these methods for measuring cognitive load, including:                some of the methods are intrusive and disrupt the normal flow of performing the task;        some of the methods are physically uncomfortable for the user;        cannot be conducted in real-time as they are too labour-intensive;        the data quality is potentially unreliable outside laboratory conditions; and        the data quality can be affected by outside factors, such as user's stress level.        
Objectively quantifying cognitive load that can be applied uniformly across fields as a standard, or to allow comparison between subjects remains an open problem. In fact, historically, the most consistent results for cognitive load assessments have been achieved through self-rating subjective measures. These allow users to describe in fine detail their perceived level of cognitive load induced by various types of task.